It’s difficult to pinpoint just one answer. I believe that you should be well-prepared about your REAL strong points. After all, even if you get hired, but your future employer is not satisfied with your performance, you will still get sacked sooner or later.

Brainstorm all points that come into your mind. Arrange them according to their importance and relevance, and practice answering this questions a couple of times.

It may also be a good idea to video yourself as you are practicing the questions. You will be amazed to see how your perception of you is different from reality. Then adjust your facial expression, gestures, postures, etc.